There are known feed forward systems used to reduce distortion in amplifiers, by a two-step method involving adjustments of the vector modulator in the nulling loop, and adjustment of the vector modulator in the error loop, some using a pilot reference signal. These methods use a correlation technique which seeks to maximize the signal correlation before elimination of the desired signal, so as to provide minimum correlation after cancellation.
Examples of the above methods are described in U.S. Pat. No. 4,389,618 to Bauman, and U.S. Pat. No. 5,576,659 to Kenington et al. The disadvantage of the first is that for adaptation of the system parameters, complex correlators that operate at RF are required for the synchronous detection, while the disadvantage of the latter is that is requires an additional RF Signal Generator for a reference signal, on which the adaptation is based. The result is that prior art systems require cumbersome and expensive apparatus.
In U.S. Pat. No. 5,548,838 to Talwar, there is disclosed an interference cancellation system employing a polar vector modulator, which operates to adjust the amplitude and phase of a reference signal, which when summed with a received signal, acts to eliminate an interfering signal component of the received signal.
Another reference containing descriptions useful in linearization system design is entitled, "Optimal feed forward linearization system design", Eid E. Eid et al., Microwave journal, Nov. 1995.
While the feedforward method is the most effective for the linearization of power amplifiers, there remains the problem of developing an adaptive adjustment of phase and amplitude controllers needed to suppress the distortion components, and in the prior art, this is usually complicated by the need for additional RF reference or pilot signals.
Therefore, it would be desirable to provide a simple method of adaptive adjustment for use in a feedforward linearization system.